1. Heat butter and oil in a saucepan, add onions and a pinch of salt, and cook until softened and golden. Add ginger and carrots and sauté a few minutes more. Add stock or water, clementine juice, salt and pepper. Bring to a boil, then simmer until carrots are tender, about 20 minutes. 2. Strain into a pitcher; put solids into a blender with 1-2 ladles of strained liquid, then purée, adding extra liquid if necessary. When smooth, add remaining liquid and purée again. 3. Reheat if necessary, taste and adjust seasoning, then serve soup in bowls and top with lime and clementine zest.